As a conventional data transmission method, there is a method for transmitting data while converting data symbols into signal levels of an electric signal or an optical signal. During such data transmission, when the signal being transmitted continuously takes a constant signal level or when it repeatedly takes a specific pattern of signal levels, a data reception apparatus cannot detect the magnitudes of original signal levels, and therefore, it cannot perform accurate data reception. As a method for avoiding this problem there is a scrambling method if which data symbols to be transmitted are evenly assigned to all signal levels by adding random numbers to digital data to be transmitted.
FIG. 8 is a diagram illustrating the construction of a data receiver for CATV (CABLE TELEVISION). As shown in FIG. 8, the data receiver for CATV is provided with a tuner 801, a down converter 802, an A/D converter 803, a detector 804, a digital filter 805, an evaluation unit 806, a descrambler 807, a clock reproduction unit 808, and a gain detector 809.
Since CATV data are scrambled, all signal levels are almost evenly included in received data symbols. Usually, quadrature amplitude modulation such as 16 QAM or 64 QAM is employed as a CATV transmission method, and a coaxial cable is employed as a transmission path.
Next, the operation of the CATV data receiver so constructed will be described.
A QAM signal received through a coaxial cable is frequency-converted through the tuner 801 and the down converter 802, and one channel of QAM signal is input to the A/D converter 803. This one channel of QAM signal is converted into a digital signal by the A/D converter 803, and an I-axis signal and a Q-axis signal are separated and demodulated by the detector 804. After removal of noises through the digital filter 805, data symbols are decoded by the evaluation unit 806, and descrambled by the descrambler 807 thereby obtaining data. Further, the clock reproduction unit 808 establishes synchronization by detecting clock shift in a transmission unit from the I-axis signal and the Q-axis signal.
Since the data are scrambled, all signal levels are almost evenly included in the received signal. The gain detector 809 detects the received signal level, and performs gain control so that the signal level is correctly decoded by the evaluation unit 806. That is, the gain detector 809 feeds the received signal level back to the down converter 802 so as to correlate the signal level to a stored threshold value, thereby performing gain control. Further, the down converter 802 performs gain control for the tuner 801 as desired.
However, when performing scrambling or descrambling, a data frame for adjusting data timing is needed, and therefore, scrambling cannot be performed on data in a format having no data frame. Moreover, even when scrambling can be carried out, if a data pattern to be transmitted matches a random number sequence used for scrambling, the same signal level continues undesirably.
Furthermore, since, in the conventional data transmission method, it is necessary to receive all signal levels to perform gain control, the data should be scrambled to be averaged. However, when a pattern of signal in which only a specific signal level appears continues, gain control becomes impossible, and accurate evaluation of signal levels cannot be carried out.
Moreover, also in a network in which plural stages of data transmission/reception apparatuses each having a transmission unit for transmitting data and a reception unit for receiving data are connected in a ring shape, the reception unit of each data transmission/reception apparatus has the same problem as in the above-described data receiver.
The present invention is made to solve the above-described problems and has for its object to provide an initialization method and a data transmission apparatus, which realize accurate data transmission independent of signal levels and patterns, without scrambling data to be transmitted, in multi-valued transmission.